Anal. Chem. 1984, 56, 1411-1416
supposed and that in certain circumstances the surface area may be the dominant factor in determining the PA signal amplitude for powdered neat samples. For a sample adsorbed onto inert support, the signal intensity will be determined by such factors as the extinction coefficient of the adsorbate, the size of the adsorbate particle, the thermal properties of the adsorbate particle, and the particle size of the adsorbent. Registry No. Potassium chromate, 7789-00-6; potassium ferricyanide, 13746-66-2.
LITERATURE CITED (1) Rosencwaig, A. “Photoacoustics and Photoacoustic Spectroscopy”; Wiley: New York, 1980.
1411
(2) Rosencwaig, A.; Gersho, A. Sclence 1975, 190, 556-557. (3) Helander, P.; Lunstrom, I.; McQueen, D. J. Appl. Phys. 1980, 51 (7), 384 I -3847. (4) Freeman, J. J.; Friedman, R. M.; Reichard, H. S. J. Phys. Chem. 1980, 8 4 , 315-319. (5) Burggraf, L. W.; Leyden, D. E. Anal. Chem. 1981, 53, 759-764. (6) King, D.; Davidson, R. S . ; Phillips, M. Anal. Chem. 1982, 5 4 , 2191-2194. (7) Kortum, G. “Reflectance Spectroscopy”; Springer-Verlag: West Berlin, 1969. (8) Bell, A. G. Philos. Mag. 1881, 7 1 (5), 515.
RECEIVEDfor review September 12,1983. Accepted February by the SERC and the This work was 13, Foundation.
Poly(ethy1ene maleate)-Cyclopentadiene: A Model Reactive Polymer-Vapor System for Evaluation of a SAW Microsensor Arthur Snow and Hank Wohltjen*
Naval Research Laboratory, Chemistry Division, Code 61 70, Washington, D.C. 20375
A model reactlve polymer-vapor system based on the DlelsAlder reaction has been used to verlfy the behavlor of a surface acoustlc wave (SAW) devlce chemlcal microsensor. Poly(ethy1ene maleate) (PEM) was syntheslzed and characterlzed both spectroscoplcally and thermally for use as a selectlve receptor of cyclopentadlene vapor. The formatlon of a nonreverslble Dlels-Alder adduct between the PEM fllm coatlng a SAW delay line osclllator and the cyclopentadlene resutts In a fllm mass change which Is easlly measured by the SAW device. Physlcal absorption of nonreactlng solvent vapors can be dlstlngulshed from the cyclopentadlene Interactlon. The resultlng cyclopentadlenedosimeter Is capable of detectlng 200 ppm of vapor In 1 mln. Slgnlflcantly lower detactlon llmtts are possible If longer vapor Integration perlods are allowed.
Surface acoustic wave devices are attractive for chemical microsensor applications because of their small size, low cost, and sensitivity. First reported in 1979 ( I ) , SAW devices have subsequently been used by several groups to construct sensors for various chemical vapors (e.g., ref 4-6). Vapor sensors based on SAW technology require a coating film which is sensitive and selective for the vapor to be detected. SAW microsensors consist of a small slab of polished piezoelectric material on which two sets of interdigital microelectrodes have been microfabricated. Typical devices range in size from less than a square millimeter to several square centimeters. When a set of interdigital electrodes is excited with a radio frequency (rf) voltage, a mechanical Rayleigh surface wave is generated. This wave is then free to propagate across the surface until it is “received” by the other set of electrodes and is converted back into an rf voltage. Connection of these two sets of electrodes together through an rf amplifier (Figure 1) permits the device to oscillate at a resonant frequency determined by the interdigital electrode spacing and the Rayleigh wave velocity. Coating of the device with a thin (e.g., 1pm thick) organic film results in a substantial reduction of the Rayleigh wave velocity and a corresponding decrease in the resonant frequency of the device. Vapors which can adsorb or absorb into the coating will alter the mass and
mechanical properties of the coating thereby producing easily measured frequency shifts. It has been shown ( 2 , 3 )that the resonant frequency shift produced by a thin coating film can be described by the following relationship:
where Af is the change in SAW resonant frequency due to perturbation of the Rayleigh wave velocity by the thin overlay f i i . Parameters kl and kz are material constants for the SAW substrate, f o is the unperturbed resonant frequency, h is the thickness of the film, p’ is the film density, V , is the unperturbed Rayleigh wave velocity, A’ is the Lam6 constant, and p’ is the modulus of the overlay film. It should be noted that the product of hp’ is simply the mass per unit area. Thus, theory predicts that the SAW frequency shift depends primarily on two factors; the mass per unit area and the mechanical properties (i.e., Lame constant and modulus) of the film. For typical organic films (e.g., polymers) the modulus is small enough to permit the mechanical property terms to be neglected thereby allowing 1 to be simplified to
Af
E
(k, + kz)f&p’
(2)
In the case of a typical 1pm thick (h = 1X lo4 m) polymer film (p’ = 1000 kg/m3) coated onto a YX quartz (k, = -9.33 X kz = -4.16 X lo4 m2 (s/kg)) SAW delay line oscillator resonating at 31 MHz, equation 2 predicts a frequency shift of -129.6 kHz. Frequency shifts of this magnitude are observed experimentally for various polymeric films (3). Application of films which are too thick result in excessive attenuation of the Rayleigh wave energy and a subsequent quenching of oscillation. It has been found that coating thicknesses approximating 1% of the acoustic wavelength are desired so that the mass absorbtion capacity for vapors will be high enough for good sensitivity but not so thick that quenching of the oscillation occurs. At 31 MHz (SAW wavelength = 100 pm on ST-Quartz) film thicknesses of about 1 pm meet this criterion. The objective of this study is to observe the behavior of a SAW vapor sensor coated with a film having a high degree of selectivity for a particular vapor. Information regarding
Thls article not subject to U.S. Copyright. Publlshed 1984 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984 PIEZOELECTRIC
Flgure 1. Surface acoustic wave delay line oscillator for chemical vapor sensing. Coatings are applied to the surface where they can perturb the Rayleigh surface wave. Wave amplitudes are nominally
i o A.
sensor response time, sensitivity, minimum detectable quantity, reversibility, and linearity obtained in this investigation is required to assess the potential of the SAW vapor sensing technique. The selective coating-vapor combination used as a model system in this study is based on the Diels-Alder reaction which occurs between a poly(ethy1ene maleate), PEM, coating and cyclopentadiene vapor.
The selection of this model reactive polymer-vapor system is made on the basis of several factors. The PEM structure is closely related to maleic anhydride which is a very reactive dieneophile. Cyclopentadiene is a very volatile compound and reacts vigorously with maleic anhydride. The cis double bond as well as the ester linkage in the polyester chain impart a soft, rubbery character to the polymer which should permit rapid permeation of vapors. Many Diels-Alder adducts undergo a retro-Diels-Alder reaction which would allow reversible operation of the detector. Finally, the simple chemical structure facilitates spectroscopic interpretation of structural changes. The exposure of a PEM thin film to cyclopentadiene vapor should result in several changes. First the film should experience a small mass increase due to the physical permeation of cyclopentadiene vapor into the polymeric matrix. Second, the film should be plasticized by the vapor permeation resulting in a reduced elastic modulus of the film. Third, reaction between the film and cyclopentadiene which has permeated should result in the formation of the Diels-Alder adduct. The reaction should cause a continuous increase in the film mass (and a corresponding decrease in SAW oscillator frequency) until all reactive sites are consumed. The approach used in this study is to synthesize and characterize PEM and verify that the Diels-Alder addition of cyclopentadiene under heterogeneous vapor-solid conditions proceeds in a rapid and near quantitative manner. Reactions of maleic anhydride, MA, and dimethyl maleate, DMM, model compounds with cyclopentadiene are conducted to verify the spectroscopic analysis of formation of the poly(ethy1ene maleate)-cyclopentadiene adduct, PEM-CP. The PEM coated
SAW device is evaluated by measuring the frequency changes caused by exposures to several concentrations of cyclopentadiene and other vapors of varying solubility parameter. The solubility parameter match between the PEM and vapor is used to develop a relation between frequency response and nonreactive vapor absorption. h final experiment is conducted by using a poly(isoprene) film with N-phenylmaleimide mixed into it, in order to ascertain the importance of having the selective receptor covalently bonded to the film matrix as opposed to having the receptor merely occluded in the film.
EXPERIMENTAL SECTION Reagents. Maleic anhydride (Eastman) was recrystallized from chloroform followed by sublimation under nitrogen at 1 atm. Ethylene glycol (Fisher) was dried immediately before use by azeotroping with toluene under nitrogen. Xylene (Fisher) was aldo dried immediately before use by azeotroping with toluene under nitrogen. p-Toluenesulfonic acid (Fisher) was used as received. Cyclopentadiene was prepared by cracking dicyclopentadiene at 170 "C under nitrogen (7). Dimethyl maleate (Pfaltz and Bauer) was distilled under nitrogen. Solvents, unless specified otherwise, were certified reagent grade and used as received. Apparatus. Infrared spectra were obtained with a PerkinElmer 267 grating infrared, spectrophotometer from solvent evaporated film samples supported on NaCl disks or from KBr pellets. NMR spectra were obtained with a Varian EM390 NMR spectrometer from deuteriochloroformsolutions referenced against tetramethylsilane. Differential scanning calorimetry and thermogravimetric analysis thermograms were obtained with a Du Pont 910 DSC, 951 TGA, and 990 thermal analysis unit under nitrogen atmosphere at a 10 OC/min heating rate. Molecular weight measurements were obtained with a Wescan 232-4 vapor pressure osmometer. All of the observations reported here were made with a SAW device designed for operation at 31 MHz. The device was fabricated on an ST quartz substrate (1.27 cm wide and 5.08 cm long) one side of which was polished (Valpey-Fisher, Inc., Hopkinton, MA). The interdigital electrodes consisted of 50 finger pairs of 2000 A gold on 200 A chromium. Each finger was 25 pm wide and spaced 25 pm from the adjacent finger. The finger overlap length was 7250 pm. The IDTs were fabricated by using standard microfabrication procedures employing an optical lithographic mask exposure of Shipley AZ-1350J photoresist followed by chlorobenzene soak, development, metal evaporation, and liftoff of the stencil and unused metal. The center to center spacing between the IDT's was 4.0 cm. A narrow bead of silicon adhesive was applied to each end of the device to eliminate triple transit echos. Electrical connections to the IDTs were made with metal pressure clips. The rf amplifier consisted of a simple two-transistor class A amplifier (3). Vapor exposures of the coated SAW device were conducted in a 2000-mL, three-neck, round-bottom flask which was fitted with vaportight glass stoppers (exposed rubber must be avoided since it results in organic vapor absorption and subsequent slow evolution which causes base line drift), electrical feedthroughs, and a stirring bar as shown in Figure 2. Samples were introduced in liquid form by a microliter syringe. The output of the SAW oscillator was connected to a Fluke Model 1910A frequency counter having a resolution of 0.1 Hz, interfaced to an Apple I1 microcomputer system which monitored and stored all experimental data. Poly(ethy1ene maleate) Synthesis. Poly(ethy1enemaleate), PEM, was prepared by an acid catalyzed polyesterification. To a four-neck, 100-mL resin kettle fitted with a nitrogen inlet bubbler, thermometer, and a Dean and Stark trap were added 15.52 g (0.250 mol) of ethylene glycol, 25 mL of xylene, and 6 mL of toluene. The kettle was heated by a mantle until 7 mL was collected in the Dean and Stark trap. The mixture was cooled t o 50 "C and 24.51 g (0.250 mol) of maleic anhydride and 0.05 g of p-toluenesulfonicacid were added. The mixture was refluxed under nitrogen for 11h. Collection of water indicated conversion was nearly quantitative. The polymer mixture was cooled to 120 "C, 0.01 g of hydroquinone was added and mixed to stabilize the unsaturated bonds, and the hot polymer was poured into a wide mouth jar. The PEM was purified by dissolving in chloroform
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
TO FREQUENCY COUNTER
Flgure 2. SAW device vapor exposure apparatus.
and precipitating into diethyl ether. When this operation is done at room temperature, the polymer separates as a gum. Vacuum drying results in a large volume of foaming which collapses on removal of the vacuum. If the precipitation is carried out below -20 OC, the PEM separates as a flocculant precipitate from which the supernate can easily be decanted. On warming the PEM becomes slightly gummy; however, much less foaming is observed on vacuum drying. The PEM should not be heated or partial chloroform insolubilization results. Diels-Alder Adducts. The maleic anhydride-cyclopentadiene adduct, MA-CP, model compound was prepared by simply adding an excess of cyclopentadiene to 0.50 g of maleic anhydride in a 25-mL Erlenmeyer flask with external cooling (7). The product was recrystallized from ethanol, mp 165 "C. The dimethyl maleate-cyclopentadiene adduct, DMM-CP, model compound was prepared by introducing 7.2 g (0.050 mol) of dimethyl maleate and 3.3 g (0.050 mol) of cyclopentadiene to the test tube liner of a 250-mL Parr bomb which was then charged with 1250 psi nitrogen and heated at 75 "C with magnetic stirring for 24 h. The reaction mixture as worked up by distillation at reduced pressure, and a fraction collected at 90-91 "C/l torr was >98% pure by gas chromatography analysis. NMR and IR spectra were consistent with the assigned structure. This adduct could not be synthesized in good yield in an open system by refluxing in chloroform solution due to competitive formation of the cyclopentadiene dimer and escape of the cyclopentadiene. The N phenylmaleimide-cyclopentadiene adduct, PMI-CP, was prepared by adding an excess of cyclopentadiene (2.00 g) to 1.00 g of N-phenylimide in a 25-mL Erlenmeyer flask with external cooling. The produce was recrystallized from methanol, mp 143 OC. While the reaction of cyclopentadiene with dimethyl maleate is slow, reaction with PEM is quite fast. The poly(ethy1ene maleate)-cyclopentadiene adduct, PEM-CP, is rapidly formed on mixing 0.50 g of PEM in 3.0 g of chloroform with 1.0 g of cyclopentadiene. The PEM-CP is isolated and purified by dropwise precipitation into diethyl ether cooled below -20 "C under nitrogen followed by decantation and vacuum drying. Further purification is obtained by reprecipatation. Molecular Weight Measurements. PEM and PEM-CP adduct molecular weight measurements by vapor pressure osmometry were made in 1,2-dichloroethane solution over a concentration range of 4 to 30 g of polymer/kg of CHzClCH2C1 solution at 35 OC using benzil as a calibration compound. Both PEM and PEM-CP occlude residual quantities of CHC13from the purification procedure, and it was found necessary to do a final reprecipitation from CH2ClCHzC1to obtain a constant and reproducible molecular weight.
1413
Solubility Parameter Determination. The PEM solubility parameter was obtained by gravimetricallymeasuring the swelling of a 30 to 40 mg quantity of PEM immersed in solvents of varying solubility parameter. Solvents included water, methanol, ethanol, 2-propanol, 2-methyl-2-propano1, acetone, methylene chloride, tetrahydrofuran, benzene, ethyl acetate, carbon tetrachloride, cyclohexane, diethyl ether, and n-pentane. PEM Film Preparation. Thin PEM films supported on SAW devices or on blank 2.5 X 7.5 cm microscope slides were prepared by spin coating at 1900 rpm. Solutions of PEM in chloroform ranging in concentration from 0.03 to 0.05 g PEM/g solution yield films ranging from 0.5 to 2.0 bm in thickness as determined gravimetrically from the blank microscope slide. The PEM density is 1.353 g/cmS. SAW Vapor Sensor Evaluation. Films of PEM were applied by spin coating onto a polished ST-quartz SAW device. PEM films having a thickness of 0.69 bm were obtained by spinning at 1900 rpm for 1min from a chloroform solution having a concentration of 0.0317 g of PEM/g of solution. The devices were baked at 110 "C for 5 min to drive off residual solvent. Films produced in this way reduced the resonant frequency of the SAW delay line oscillator by approximately 90 kHz when compared to an uncoated SAW device (fo= 31.346 800 MHz). Coated devices were allowed to stand in clean air for at least 12 h prior to use since this procedure seemed to eliminate drift problems encountered when freshly coated devices were used immediately. Films of cis-poly(isoprene)mixed with N-phenylmaleimide were prepared on the SAW devices using a different procedure. Several drops of a chloroform solution containing N-phenylmaleimide and cis-poly(isoprene) in a 1:1ratio were coated onto the device and the solvent was allowed to evaporate. As before, the device was baked at 110 "C for 5 min to drive off residual solvent. Frequency shifts of the SAW oscillator caused by vapor interaction with the coating film were obtained by injecting the sample into the three-neck flask in liquid form using a microliter syringe. The volume injected determined the vapor concentration after complete volatilization had occurred. A magnetic stirring bar was used to ensure homogeneity of the vapor mixture. All experiments were conducted at room temperature (25 A 2 "C) and atmospheric pressure. RESULTS AND DISCUSSION Since this PEM coating-cyclopentadiene vapor model system is being used to obtain operational information and limitations of the SAW device as a vapor sensor, a detailed characterization of the chemical system was undertaken. This will allow correlation of the coating structure and coatingvapor interactions with the SAW device response. The characterization includes spectroscopic and thermal analyses, molecular weight measurements, and solubility parameter determination. The SAW device response involves time-dependent frequency response measurements of the PEM coated device to vapor exposures. Infrared and NMR Spectra. Infrared and NMR spectra of PEM and of PEM-CP adduct are consistent with assigned structures and display a good correspondence with analogous changes in the infrared and NMR spectra of the MA and DMM systems when converted to their cyclopentadiene adducts. An identical infrared spectrum of the PEM-CP adduct was obtained by simply evaporating a thin PEM film from chloroform solution onto an NaCl disk followed by a 10-min exposure to a saturated cyclopentadiene atmosphere. This indicates that the adduct forming reaction is rapid and quantitative. Heating of the PEM-CP adduct film to temperatures as high as 185 "C did not provide an infrared spectrum indicative of the original PEM film. The spectral changes observed were characteristic of degradation. Molecular Weight. It was of interest to obtain a molecular weight measurement on this system as well as use the measurement to determine the degree of conversion to the Diels-Alder adduct. When PEM reacts with cyclopentadiene to form the adduct, a molecular weight increase proportional to the PEM-CP to PEM repeat unit molecular weight ratio
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1984
eE 6 0 1 ~
a
SOLUBILITY PARAMETER 8 (cal/cm3)''2 1
0
I
I
100
I
1
200
I
I
300
I
4 00
TEMPERATURE ("C)
Figure 4.
r
Flgure 3. DSC and TGA thermograms of poly(ethy1ene maleate) and the poly(ethy1ene maleate)-cyclopentadiene adduct.
would be predicted assuming a quantitative conversion. The PEM number average molecular weight is 4520 which corresponds to 32 repeat units. The molecular weight of the PEM-CP adduct is 8250 which corresponds to 39 repeat units assuming quantitative addition of the cyclopentadiene to the PEM repeat unit. A possible source of greater than 1:l stoichiometry would be addition of a second cyclopentadiene unit to some of the PEM-CP repeat units although we do not have other data at this time to support such a reaction. Another source would be a partial fractionation of the PEMCP adduct to higher molecular weights during the precipitation purification operation to separate the polymer adduct from absorbed cyclopentadiene and dicyclopentadiene byproduct. In a simple gravimetric experiment, where a thin film of PEM coated onto a microscope slide was exposed to a saturated atmosphere of cyclopentadiene followed by evacuation, the ratio of exposed film weight to initial film weight was 1.25. If the uptake is quantitative, this ratio should correspond to the ratio of the PEM-CP repeat unit to the PEM repeat unit molecular weight which is 1.47. Thermal Analysis. Thermal analysis data were obtained to determine the change in glass transition temperature effected by PEM-CP adduct formation, and, if the adduct formation were reversible, at what temperature the retroDiels-Alder reaction would occur. The DSC and TGA thermograms of PEM and PEM-CP are presented in Figure 3. Conversion of PEM to PEM-CP results in only a small Tg increase from -10 to 0 "C. At higher temperatures the PEM exhibited a weak endotherm at 125 "C while the PEMCP adduct displayed an exotherm at 150 "C. TGA indicated this 150 "C exotherm did not correspond to a rapid liberation of cyclopentadiene. When a sample of the PEM-CP adduct was heated to 185 "C in the well of an evacuated IR gas cell, only a trace of cyclopentadiene vapor was observed. This is consistent with the infrared experiment involving heating a PEM-CP adduct film as discussed earlier. Similar analysis on the maleic anhydide model system showed a reverse Diels-Alder reaction while the dimethyl maleate system followed the behavior of the PEM-CP adduct. Solubility Parameter. The solubility parameter was measured to determine the variation in PEM physical absorption of solvents and whether the selectivity would be parallel by vapor phase detection measurements with the SAW device. The swelling data are presented in Figure 4. The PEM solubility parameter is 9.7 f 0.5 ( ~ a l / c m ~ )and ~ ' ~the , more polar solvents outside of this range are more strongly
Poly(ethy1ene maleate) solubility parameter determination
by solvent swelling measurements.
P 120 t
0
5
0
/
1
3 Dz
U
U
0 w
c
2 Y
I
I
I
I
I
2000
4000
6000
8000
10000
CYCLOPENTADIENE CONCENTRATION (pprn)
Response of PEM-coated SAW device to various cyclopentadiene vapor concentrations.
Flgure 5.
absorbed than the less polar solvents. PEM Coated SAW Device Response to Cyclopentadiene Vapor. Injection of cyclopentadiene into the closed flask containing the coated SAW device resulted in a reduction of the oscillator frequency with time. Increasing the concentration of cyclopentadiene by repeated injections into the flask resulted in corresponding increases in the rate of change of the oscillator frequency. For example, at a concentration of 4000 ppm of vapor, the SAW device frequency dropped at a rate of 76.4 Hz/min. After 10 min at this concentration an additional injection of cyclopentadiene was made to increase the vapor concentration to 5000 ppm whereupon the rate of frequency change increased to 89.9 Hz/min. The rate at which the SAW device frequency decreased when exposed to various concentrations of cyclopentadiene is shown in Figure 5. The response is linear within experimental error over the concentration range investigated (i.e., 100 ppm to loo00 ppm). The rate of the SAW frequency decreases is directly proportional to the rate of increase of mass of the coating film which is related to the Diels-Alder adduct formation rate. Thermal studies indicated that the retroDiels-Alder reaction did not occur significantly even a t elevated temperatures. Thus, the sensor was considered to be completely irreversible. When the system was flushed with clean air it was observed that the SAW frequency increased, indicating a significant loss of weight by the film. Continued
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
flushing resulted in a stable SAW frequency which typically was about 40% less than the maximum frequency shift observed. For example, if the total cyclopentadiene exposure had shifted the SAW device resonant frequency down by 10 kHz, then flushing with air could cause the frequency to increase by about 4 kHz. It is believed that this partial reversibility is a result of outgassing of cyclopentadiene which had permeated into the regions of the PEM film where the Diels-Alder adduct had already formed. The slope of the curve describing the rate of frequency change vs. cyclopentadiene concentration was -0.019 Hz/ (ppm min). The average base line noise observed in these experiments was less than 2 Hz RMS measured over a 1-min interval. Thus,the minimum detectable quantity for a 2 1 signal to noise ration and a 1-min observation time is about 200 ppm. However, the nonreversible nature of the Diels-Alder adduct can be used to extend the detection limits much lower by increasing the exposure time. In this way the film behaves as a chemical dosimeter, integrating the concentration of cyclopentadiene exposure with time. While quantitative experiments to explore this possibility were not conducted, a qualitative observation of this effect was often made when the sample flask was not adequately cleaned after use. Early in these investigations, the sample flask was cleaned after being exposed to cyclopentadiene by rinsing with acetone followed by soap and water washing. Although clean to the sense of smell, reuse of the flask resulted in a steady downward base line drift over a period Qf hours. The drift could only be eliminated by heating the flask in a Bunsen burner prior to use. Thus, long term exposure to low concentrations of vapor results in a continuous, measurable mass increase of the film. If one assumes that a film whose mass produces a frequency shift of 100 kHz on a 31-MHz SAW oscillator can pick up 20% of its own mass in cyclopentadiene (i.e,, 20 kHz) before it is exhausted, then the estimated life of such a dosimeter is in excess of 1700 h when exposed to concentration of cyclopentadiene around 10 ppm. PEM Coated SAW Device Response to Various Vapors. The PEM-coated SAW device was exposed to a variety of vapors to determine the selectivity of the film. Aliquots measuring 2000 ppm of vapor were prepared in the sample flask by syringe injection of volatile liquid and the response of the SAW oscillator frequency with time was recorded. The flask was purged with clean air for approximately 10 min after the sensor response leveled off. The cycle of injection, equilibration, and purge was repeated for samples of acetone, methylene chloride, benzene, methanol, pentane, and cyclopentadiene. The results are illustrated in Figure 6. All of the samples investigated except cyclopentadiene caused a response which leveled off quite rapidly after injection. The device response after exposure to cyclopentadiene never leveled off. Upon purging the sample flask the device frequency stopped its steady downward treed and returned to a level which was significantly below that at which it started. The results obtained from solvent vapor exposures are consistent with those expected of nonreactive vapors which permeate the film. The concentration of vapor in the film quickly reaches equilibrium with the concentration of vapor in the flask. Removal of the vapor in the flask allows the removal of vapor from the film. Thus, the reversible SAW frequency shift observed is simply a result of the physical sorbtion of the vapor causing an increase in the effective mass of the film. The device response to cyclopentadiene vapor exhibits both reversibly and nonreversible components. As described previously, the reversible component of the response is probably due to unreacted cyclopentadiene outgassing from the film. The formation of the Diels-Alder adduct is responsible for the nonreversible component of the response.
1415
I ( X 103) TIME (SECONDS)
Flgure 6. Response of PEM-coated SAW device to 2000 ppm exposures of various vapors.
It is well known from chromatography (see, for example, ref 10) that a volatile solute vapor can dissolve in a nonvolatile solvent coating. The resulting equilibrium can be described by a partition coefficient for a nonideal solution (IO) K = - RT
(1)
MlYP2
where K is the partition coefficient defined as the ratio of the weight of solute vapor per gram of solvent coating to the weight of solute vapor per cm3 of vapor volume at the system temperature, R is the gas constant, T is the temperature of the system, MI is the molecular weight of the solvent coating, p z is the vapor pressure of the solute gas at the system temperature, and y is the activity coefficient for the solute/solvent system. For regular solutions, the activity coefficient is related to the solubility parameters of the solute and solvent through the following relationship (11):
where 62 is the solubility parameter of the solute vapor, 61 is the solubility parameter of the solvent film, and is molar volume of the vapor. This can be combined with eq 1to yield
v2
While this expression is rigorously correct only for regular solutions, it does offer significant qualitative insight even when the solvent is a permeable polymeric film. If one is interested in comparing the relative magnitudes of the partition coefficient for various vapors in the same coating film and at the same temperature, then eq 3 can be simplified to
since R, T, and M I are constants. The values for the relative partition coefficient for the various vapors investigated are tabulated along with the experimentally measured frequency shift in Table I. A system temperature of 300 K and a solubility parameter of 9.7 ( c a l / ~ m ~ )for * / PEM ~ are assumed. Vapor pressures were obtained from ref 9. Solubility param-
1418
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
Table I. Measured Frequency Shifts and Calculated Relative Partition Coefficients for Various Vapors in PEMa
acetone methylene chloride benzene methanol n-pentane a
10.0 9.7 9.2 14.5 7.0
-166 (1.0)
3.4 (1.0)
-117 ( 0 . 7 )
1.6 (0.47) 9.0 (2.6) 3.0 (0.88) 0.4 (0.12)
-162 (0.98) -137 (0.83) -16 (0.10)
Values in parentheses are normalized to acetone.
eters were obtained from ref 8. The results in parentheses presented in Table I have been normalized to acetone to allow an easier comparison. Calculated values for the relative partition coefficient predict that the following order of frequency shift should be observed: benzene > acetone > methanol > methyl chloride > n-pentane. Experimentally, the following order of frequency shifts was observed: acetone > benzene > methanol > methylene chloride > n-pentane. Agreement between theory and experiment is qualitatively and quantitatively good except for benzene. The reason for this discrepancy is not known. Poly(isoprene)-Phenylmaleimide Coated SAW Device Response. To determine whether chemical bonding of the vapor reactive functional group to the polymer matrix was required for selective detector response, a simple dienophile was occluded in a cis-polyisoprene matrix and tested for cyclopentadiene sensitivity. Maleic anhydride was not suitable for this experiment since it evaporated from a 50 wt YO polyisoprene f i i in 20 min. Substitution of N-phenylmaleimide, which has Diels-Alder reactivity comparable to maleic anhydride, solved the evaporation problem. However, the spin coating technique could not be used to apply the N-phenylmaleimide occluded polyisoprene film. Spectroscopic analysis of films prepared in this manner showed that no N-phenylmalemide was retained in the film. This was confirmed by the lack of a detector response of the SAW sensor. When a film was prepared by evaporation of a 1:l N-phenylmaleimidepolyisoprene solution in chloroform, a detector response of magnitude comparable to the PEM film was obtained. This experiment indicates that covalent bonding of the vapor receptor of moiety to the polymer matrix is not necessary for
detector sensitivity. However, it is necessary if the spin coating technique of application is used. CONCLUSION A SAW delay line oscillator coated with a film of poly(ethylene maleate) has been shown capable of detecting cyclopentadiene vapor in a highly selective and sensitive manner. Both the sensitivity and selectivity of the microsensor are determined primarily by the coating film. In this study, the selectivity was high due to the specificity of the Diels-Alder reaction between the cyclic diene vapor and the dienophile coating. Sensitivity was determined largely by the rate at which the vapor reacted with the coating since the reaction was irreversible. Coating films which react more rapidly are desirable so that low concentrations of vapor will provide a larger mass change in a shorter period of time. Thus, the dosimeter would be able to sense the presence of lower vapor concentrations for a given exposure time. ACKNOWLEDGMENT The authors wish to thank N. L. Jarvis of the Naval Research Laboratory for helpful discussions during the course of this work. Registry No. PEM (SRU),25949-13-7; PEM (copolymer), 26009-59-6; CP, 542-92-7; MACP, 826-62-0;DMMCP, 5826-73-3; PMI-CP, 26234-46-8; PEM-CP, 25949-13-7; acetone, 67-64-1; methylene chloride, 75-09-2; benzene, 71-43-2;methanol, 67-56-1; n-pentane, 109-66-0. LITERATURE CITED (1) Wohitjen, H.; Dessy, R. E. Anal. Chem. 1979, 5 1 , 1458. (2) Auld, B. A. “Acoustic Fields and Waves in Solids, Voi. 11”; Wiiey-Interscience: New York, 1973; Chapter 12. (3) Wohltjen, H. NRL Memo. Rep. 1984, No. 5374. (4) Fertsch, M. T.; White, R. M.; Mulier, R. S. Presented at the Device Research Conference, Ithaca, NY, 1980. (5) Bryant, A,; Lee, D. L.; Vetellno, J. F. Ultrason. Symp. Roc. 1981, IEEE Cat. 81CH1689-9, 1735. (6) D’Amico, A.; Palma, A.; Verona, E. Appl. Phys. Left. 1982, 4 1 , 300. (7) Wagner, E. C.; Hunt, W. C. J . Chem. Educ. 1951, 28, 309. (8) Karger, B. L.; Snyder, L. R.; Horvath, C. “An Introduction to Separation Science”; Why-Interscience: New York, 1973; p 273. (9) Weest, R. C., Ed. “Handbook of Chemistry and Physics”, 53rd ed.; Chemical Rubber Co.: Cleveland, OH, 1972; pp D151-Dl58. (IO) Littiewood, A. 9. “Gas Chromatography: Principles, Techniques and Applications”; Academic Press: New York, 1962; p 51. (11) Karger, 9. L.; Snyder, L. R.; Horvath, C. “An Introduction to Separation Science”; Wiley-Interscience: New York, 1973; p 52.
RECEIVED for review August 1,1983. Accepted March 5,1984.
